There has previously been proposed by the present applicant, see for example WO2013/132418, a printing system in which, at an image forming station, an aqueous ink is jetted onto an endless belt or drum that serves as an intermediate transfer member (ITM). The resulting ink image is transported by the ITM to an impression station and, during its transportation, it is dried to leave behind a tacky ink residue. At the impression station, the ink residue is transferred onto a substrate and the ITM surface then returns to the image forming station to commence a new printing cycle.
Certain problems have been encountered during operation of such a printing system to which the solution has been found to be the blowing of a gas (air) stream through the gap traversed by the ink droplets from jetting nozzles of print heads mounted on a print bar to the surface of the ITM. These problems are briefly explained below:
First, the ITM is operated at an elevated temperature and the ink droplets start evaporating on impacting the ITM. The released water vapour then condenses on the cooler print heads and forms droplets, which eventually drip onto the ITM to damage the printed image. Preventing such condensation requires a fast gas stream and, because of the turbulence that it creates, such a stream can only be applied intermittently during periods when no jetting of ink is taking place, such as between pages or between print runs.
Second, when a droplet is jetted by a printing nozzle, it is often followed, a short time after it has separated from the printing nozzle, by a much smaller droplet, referred to as a satellite. Being emitted sequentially, the droplets and their satellites do not fall on the same point on the ITM and therefore result in some image dots on the substrate having a faint shadow caused by their satellites. To overcome this problem, it has been proposed to blow a constant steady laminar stream through the gap between the ITM and the print heads. The effect of this stream is to carry all droplets in the direction of movement of the ITM. However, because of their size, the smaller satellites are more strongly affected by the gas stream than the larger droplets and if the stream speed is carefully selected, the large droplets and the satellites merge into one another on reaching the surface of the substrate.
In the following description, the laminar stream for avoiding satellites is referred to as the low speed stream and the turbulent stream for dislodging condensation from the jetting heads is referred to as the high speed stream. Furthermore, the sources for supplying these two gas streams will be referred to as high pressure and low pressure supplies but the terms “low” and “high” are used only to distinguish the stream and supplies from one another.
The present disclosure seeks to provide a manifold that is capable of delivering both types of gas stream into the small gap at the image forming station between the print heads and the ITM.